Method for transiently wetting lens molds in production of contact lens blanks to reduce lens hole defects

ABSTRACT

Hydrophilic contact lens molds are transiently modified to provide a water dynamic contact angle equal to or less than 100° by treatment with a wetting agent which may be water or a surfactant composition which provides a mold surface having a water wetting force of at least 70 mg to afford a reduction in lens hole defects in hydrophilic contact lens manufacture.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to methods for reduction of defects in, and animprovement in yield, in the production of contact lens blanks. Moreparticularly, it provides measures to control and minimize lens blankdefects known as lens holes.

Contact lens prepared from hydrophilic hydrogel polymeric materials arenow well known, and are prepared commercially in great volume in highlyautomated manufacturing facilities. As these products are intended forintimate contact with the eye, great care is taken to assure that theymeet stringent quality control standards. This can result in arelatively high reject rate, adversely affecting economies in theirproduction.

Accordingly, it is an object of the invention to control and minimizelens blank defects, specifically lens holes. Additionally, it is anobject to afford a method operable in high speed automated manufacturingoperations to improve yields adversely affected by rejects due to lensholes. Further, it is an object to provide means for reducing lens holedefects originating in the filling operation and attributable to unevendistribution of the reactive monomer mixture forming the lens blank. Itis also an object to provide a method for effecting even distribution ofreactive monomer mix about and upon the convex or backcurve mold.Finally, it is an object to modify and render relatively uniform thesurface energy of a mold surface on a temporary or transient manner i.e.without a permanent change to the mold surface.

Processes for preparing hydrogel contact lens blanks have been welldocumented. Briefly, reactive monomer mix (RMM) for the formation ofhydrophilic contact lenses is dispensed into a concave or frontcurvemold, formed from a hydrophobic polymer such as polystyrene, at afilling station. A convex or backcurve mold is then brought intoproximate engagement with the frontcurve mold to form and shape the lensblank therebetween. Next a mechanically associated assembly formed fromthe frontcurve mold/RMM/backcurve mold is traversed through a UV curingtunnel under conditions to cure the RMM. The cured lens blank productsassociated with the forming molds are then dissociated by removing thebackcurve mold. The cured lens/frontcurve assembly is then traversedthrough leaching and hydration tanks, and a contact lens is therebyproduced.

The continuous process implementing the complete manufacturing processutilizes a lens mold manufacturing zone, comprising first and secondinjection molding stations for the formation of concave and convex lensmolds, respectively, and includes a transport line upon which concaveand convex lens parts may be conveyed from zone to zone; an enclosedzone (`nitrogen tunnel`) maintained under nitrogen for degassing moldhalves or sections; a filling zone for filling concave mold sectionswith reactive monomer composition, registering concave and convex moldsections in aligned relation, and engaging same in mating moldingrelation optionally under vacuum conditions, and precuring said reactivemonomer composition with ultraviolet light to a gel-like state, and acuring zone in which the cure is completed and the finished lens blankreadied for demolding. It will be appreciated that the entire process isintegrated via transport means, generally one or more conveyors upon orin relation to which lens molds are assembled, arranged or interleavedin the course of conveyance through the said zones or stations inoperational sequence. The lens molds may for convenience be situated inor upon mini-pallets (for example, fabricated of cast aluminum,stainless steel or the like) containing a number of lens molds (forexample eight) arranged regularly thereon in spatial relation correlatedwith the treatment stations and the automated material transferequipment where employed. All of the conveyance belts or tunnels areunder nitrogen or inert gaseous blankets.

In greater pertinent detail, the concave or frontcurve moldincorporating an optical molding surface together with a peripheral zoneor flange for interactive engagement with the convex or backcurve moldis traversed through a stamping station in which the peripheral flangeportion of the mold is treated with a surfactant material withoutcontact with the optical surface of the mold, as described and claimedin copending and commonly assigned U.S. Ser. No. 08/258,263, now U.S.Pat. No. 5,542,978 of Kindt-Larsen et al. for METHOD AND APPARATUS FORAPPLYING A SURFACTANT TO MOLD SURFACES incorporated herein by reference;the mold is thereafter filled, sometimes to overflowing with reactivemonomer mix whereupon the frontcurve mold is engaged in mating relationwith the convex or backcurve mold, (the optical surface of which istypically untreated); the paired, juxtaposed mold assembly including theRMM molded therebetween, is passed to a curing zone and thence to afirst demolding station at which the mold sections are disengaged.Facilitated by the presence of the surfactant upon the peripheral flangeof the frontcurve mold, the excess material is separated from theremainder of the cured lens blank and retained with the convex orbackcurve mold; the optical portion of the lens then is retained by thefrontcurve mold, whereupon the excess waste material may be removed fromthe backcurve mold by any suitable mechanical means, wherefore thefrontcurve mold associated with the retained lens blank free of excessperipheral material is passed to leaching and hydration stations,ultimately to be demolded, the contact lenses to be collected, andprepared for shipment.

Aspects and preferred features of the contact lens manufacturing systemin part described and claimed herein are detailed in copending andcommonly assigned application Ser. No. 08/258,267, now abandoned of Lustet al. for Apparatus for Removing and Transporting Articles from Molds;application Ser. No. 08/257,786, now abandoned of Wang et al. forProduction Line Pallet System; Ser. No. 08/257,792, now U.S. Pat. No.5,441,901 of Martin et al. for Mold Clamping and Procedure of aPolymerizable Hydrogel; the application Ser. No. 08/257,785, now U.S.Pat. No. 5,540,410 of Lust et al. for Mold Halves and Molding Assemblyfor Making Contact Lenses; and application Ser. No. 08/257,802, now U.S.Pat. No. 5,498,528 of Martin et al. for On-Line Injection Molding withNitrogen Blanket, the disclosures of all of which are incorporatedherein by reference.

In the course of commercial operations including high speed productionof lenses in volume, a small member of defects can seriously affectyield, and resultant economics of the process. This is particularly thecase where, in consequence of the use of automated manufacturingequipment a defect in a single lens can result in the loss of a largernumber of lenses with which it is associated for example in the courseof being transferred via integral pallets or frames from onemanufacturing station to another.

Lens defects occur for many reasons, including simple misalignment ofmanufacturing equipment, but as the latter is readily correctablethrough engineering adjustment, interest is focussed principally on lensholes and puddles formed in the course of filling and curing steps,employing the reactive monomer mix (RMM).

Lens holes include voids i.e. areas which contain no monomer, pits i.e.areas of nonuniform thickness, and other similar regularities such asuneven edges, being a function of the efficiency of spreading of thereactive monomer mix on the surface of the convex backcurve mold whenthe two mold halves are joined.

Puddles, another lens defect, in random or tree branch shapes generallyfound along the lens edge, are generated during the curing step, and areassociated with the concave or frontcurve mold.

High speed photography has demonstrated the formation of lens holes inthe filling operation during the spreading of the advancing meniscus ofthe RMM upon the convex or backcurve mold. However, the occurrence ofthe defect is apparently indiscriminate, especially considering thenumber of sound lenses produced in the same manner on the sameequipment. It had already been established that on a macro scale, RMMwets the polystyrene mold surface well.

However, fundamental studies (based upon work reported by R. H. Dettreand R. E. Johnson Jr. J. Phys. Chem. 68, 1507 (1096) and in Surface andColloid Science, E. Matijevic, Ed., Wiley-Interscience, N.Y. 1969, 161.2pp. 85 and S. P. Wesson, TRI Progress Report #49, Textile ResearchInstitute, Princeton N.J. Aug. 23, 1992) showed that the mold surface,formed of a hydrophobic polymer such as polystyrene, was a low energyheterogeneous surface having a small portion of high energy surfacedomains. This was consistent with knowledge that the molding resins weretypically fabricated for injection molding purposes to contain certainadditives including mold release agents, which could provide the highenergy domains on the mold surface.

There was in consequence established the need for means to modify thesurface activity at the interface between the convex or backcurve moldand the reactive monomer mix, in the context of dynamic lens formationduring molding and in particular, during the original contact with theRMM, and the advancing meniscus thereof onto and across the convex mold.In particular, it was desired to establish during molding an increase inthe high energy surface area exhibited by the convex mold.

2. Discussion of the Prior Art

In order to implement the high-speed and mass-production molding of suchhydrophilic contact lenses, there have been developed two-part moldsincorporating pallet-supported mold structures; for example, asdisclosed in U.S. Pat. No. 4,640,489 to Larsen, and methods of formingshaped polymeric hydrogel articles, such as hydrophilic contact lenses,elucidated in the disclosures of U.S. Pat. Nos. 4,680,336 and 5,039,459to Larsen et al.

The release of hydrophilic contact lenses from adherent mold surfacessubsequent to the completion of the contact lens molding process can befacilitated or improved upon, as is set forth in the disclosure of U.S.Pat. No. 5,264,161 to Druskis et al. In that instance, surfactants areintroduced in solution into a hydration bath employed in the moldingcavities for molding the hydrophilic polymeric structures or contactlenses. The surfactant which is dispersed in the hydration bath inconcentrations not exceeding 10% by weight aids in facilitating releaseof the lenses from adherent contiguous mold surfaces being separated,the function of such surfactant being to reduce the surface tensionproperties of water or liquids, and to thereby reduce the level ofadherence between components consisting, on the one hand, of the contactlenses and, on the other hand, the mold surfaces which become adherentduring molding. Numerous types of surfactants are disclosed in thispatent publication, such as polymeric surfactants includingpolyoxyethylene sorbitan mono-oleates, which are especially suitable forreleasing in an undamaged state any hydrophilic polymer articles fromadherent mold surfaces which are constituted of plastic materials.

U.S. Pat. No. 4,159,292 describes the use of silicone wax, stearic acidand mineral oil as additives for plastic mold compositions to improvethe contact lens release from the plastic molds. Commonly assigned U.S.patent application Ser. No. 08/414,999 of Kindt-Larsen et al.,incorporated herein by reference, discloses the use of internalsurfactant or mold release agents to facilitate lens separation from theoptical surface of lens molds.

The use of surface applied surfactants as release agents in connectionwith the manufacturing of hydrogel contact lenses is disclosed andclaimed in copending and commonly assigned application Ser. No.08/431,612 filed May 1, 1995 by Kindt-Larsen et al. as acontinuation-in-part of application Ser. No. 08/258,263 filed Jun. 10,1994, now U.S. Pat. No. 5,542,978 each of which is incorporated hereinby reference. In that application, a thin layer or film of a surfactantsuch as Tween 80 is applied via a stamping head to surface regionsextending about, i.e., peripherally of the frontcurve of a mold part forthe forming of contact lenses, to facilitate the lens release upondemolding of all or part of the peripheral rings of RMM materialexpressed externally of the mold by virtue of overrun during filling. Inthis application, no surface active material is applied to the portionof the mold defining the optical face of the lens.

In these circumstances, primary consideration is directed to thephysical difficulty engendered by adhesion of the contact lens to one oranother of the mold surfaces, or to efficient handling of waste materialresulting from overrun of the RMM in filling. In automated operations itis significant to assure uniform reliable separation of mold components,waste, and contact lens product on a consistent basis. In consequence,efforts are centered upon the steps of demolding following filling, inwhich the concern is to assure preferential displacement of the concaveand convex molds relative to the contact lens blank and the wasteportion, if any, at the periphery of the product along the peripheralflange of the mold. In preferred manufacturing practice, the contactlens is retained by the front curve or concave mold while the wasteportion at the periphery is disengaged from the peripheral flangethereof and retained by the convex mold assembly as it is mechanicallydisengaged from molding relation. This first demolding is contrastedwith the second demolding of the cured, hydrated contact lens from thefront curve mold. Different conditions and considerations obtain ofcourse, in each of these demolding operations.

An entirely distinct problem is presented by the perceived need tocontrol the wettability of the convex mold surface to the RMM in anendeavor to reduce lens hole defects. The release characteristics of themold surface for demolding do not straightforwardly correlate to thewettability phenomenon especially under varying ambient conditions andpotential compositional variations in the RMM formula. Moreover, moldshaving a modified surface, as by internal loading with an agent such aszinc stearate, while useful in affording wettability to the RMMinterface, on the one hand can offer only a singular level of surfaceenergy domains at any one time, and if reused, may offer degradation inthat characteristic with use over time as an internal lubricant sourceis exhausted by exudation. Furthermore, the phenomena of wettability anddemolding involve fundamentally different considerations, such that asemi-permanent change in the mold surface may favorably affectwettability but unfavorably affect demolding.

In consequence it is desired to meet the requirements for enhancedwettability at the mold/RMM interface in a more flexible and responsivemanner, taking into account ambient conditions and other dynamic aspectsof the manufacturing process such as speed of the operating line, hencetime for mold wetting and demolding.

SUMMARY OF THE INVENTION

It has now been found that one may conveniently control characteristicsof the mold surfaces critical to the minimization of lens hole defectsby providing a transiently modified mold surface having an enhanced,more homogeneous population of surface domains of high energy thuslyfacilitating the wettability and spreadability of RMM across the moldsurface under conditions of high speed automated operations.

More specifically, the hydrophobic optical surface of the mold may betreated with a removable surface agent forming a transient uniformcoating thereon whereby the transfer efficiency at the RMM/moldinterface is improved for the wettability and coating of that moldsurface as the advancing meniscus of the RMM is engaged during themolding sequence. It is important to appreciate that the surfaces areengaged in mating relation about the RMM for microseconds during eachmolding cycle. Naturally, given the somewhat viscous nature of the RMMmaterial, any domain of the mold seen to lack receptivity to, i.e.wetting by the RMM will be bypassed in the dynamic spreading of the RMMupon and around the defining surface of the mold, leading to defects.

Given that the mold where reused may cycle thousands of times per houran internal lubricant such as zinc stearate, while effective in mostcases, may not over longer runs at high speeds and under differentambient conditions be capable of controlling surface characteristicsconsistently; the periodic modulation of surface characteristicsprovided by a surface coating of transient nature offers operatingadvantages and greater certainties of yield with improved qualityperformance, together with flexibility and responsiveness to ambientconditions, as aforesaid.

More specifically it has been recognized that ambient operatingconditions, in particular relative humidity, can impact the wettabilityof the mold contact surface for the hydrophilic RMM - and when employedalong or in conjunction with the use of surface applied surfactants orinternal lubricants can establish satisfactory conditions forminimization of lens hole defects. Control of this variable alone underproper conditions can also be effective.

As is well known, relative humidity commonly refers to an ambientmoisture condition in the surrounding air, which will of course varyseasonally as in response to heating or air-conditioning conditionsobtaining in the manufacturing environment. That environment itself maybe controlled or a microclimate may be created by at least partialclosure of any operating zone, in this case the filling zone of themanufacturing zone. The latter constitutes a particularly convenientapplication where a surfactant is used, conjuntively, as either or bothof a reduced humidity and reduced level of surfactant may be employed,and recovery of excess chemical agents from the enclosed zone may bemore readily achieved for better environmental control and recycling.

In general it has been found that maintenance of a relative humidity of60 to 80% in the area of the filling zone is sufficient to maintain asurface condition on the optical surface of the mold of satisfactoryreceptivity to efficient spreading of the RMM on the mold surface toachieve minimization in lens hole defects. Such a condition may beeffected as necessary by spray jets or other vaporization sourceinducing a higher level relative humidity than that obtained underambient conditions; or those conditions themselves, specifically ambienttemperature, may be manipulated to induce the desired relative humidityin a manner well known in the art. Thus the temperature in the zone maybe increased, as by the use of electric strip heaters to permit theambient air to accommodate a higher relative humidity. The temporal ortransient nature of the condition is appreciated when one recognizesthat the moisture coating is readily absorbed upon contact with thereactive monomer mix.

In a alternative embodiment described and claimed in concurrently filedAppln. Ser. Nos. 08/536,944 of Jen, et al., for Method of Reducing LensHole Defects in Production of Contact Lens Blanks, incorporated hereinby reference, the convex mold is pretreated by application of a surfaceactive agent to at least the contact face of the mold by applying, as byspraying, dipping or any other suitable means, a surfactant such asTween 80, appropriately in a solvent vehicle therefor such as water, toprovide a concentration of 0.05 to 5.0% w/w of surfactant. Thepretreatment may be applied in conjunction with i.e. to precede eachmold cycle or may be effected intermittently, to maintain the requiredsurface energy requirements for reduction of lens hold defects. Theamount of surfactant to be employed will also be gauged by a certainbalance between measures taken to ensure desired lens releasepreferentially from the convex molding surface at this stage; and lensdemolding from the concave or frontcurve mold following cure. Thus, in apreferred embodiment, described in concurrently filed and commonlyassigned application Ser. No. 08/536,760, of James Jen et al. for MOLDMATERIALS MADE WITH ADDITIVES the concave or frontcurve mold may beformed from a composition incorporating a compatible agent such as zincstearate whereas the surface of the convex or backcurve mold is treatedperiodically with a surfactant in accordance with this invention asaforesaid.

In this manner, it has been shown that lens hole defects can be reducedby as much as several percent in a high speed automated pilot productionline thereby effecting substantial economic savings while increasingefficiency of the manufacturing operation.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a graph of the % lens holes (visual) vs. water wetting force(mg) of lens holes reduction with increasing water wetting force andillustrating the preference herein for the use of wetting agentsexhibiting a water wetting force generally in excess of 70 mg.

DETAILED DESCRIPTION OF THE INVENTION

In accordance with the invention, the interfacial tension between thereactive monomer mix, more specifically the advancing meniscus thereof,and the optical surface of the convex or backcurve mold is controlledand minimized by normalizing and extending the surface areasrepresenting high surface energy domains in the convex mold contactsurface. This is accomplished by the temporal or transient applicationof a surface modifying agent to the optical surface of the convex moldin amount and characteristics effective to increase the population ofthe high surface energy areas on the convex mold surface to affordimproved wettability by the RMM and effecting a differential in surfaceenergy as between the respective mold halves, to overcome the energyrequired to separate the two mold halves so to preferentially effect theretention of the optical lens portion on the frontcurve mold.

Thus, it will be appropriate to consider the surface characteristicexpressed in and upon the frontcurve mold in applying the precepts ofthis invention, as it is desired to assure that the lens blank bedemolded preferentially from one mold surface consistently throughoutthe manufacturing process, usually (and as described herein) from theconvex mold, thereby permitting the retention of the lens blank in theconcave frontcurve mold in its precured, prepolymerized state fortraverse to and through the remaining manufacturing stages. Inconsequence, it is necessary to take into account, relative to the typeand amount of surface modifying agent to be applied to the convex moldsurface, whether the frontcurve or concave mold has itself beenpretreated, usually by an internal additive such as zinc stearate tomodify the surface characteristics of the mold, as disclosed inconcurrently filed and commonly assigned application Ser. No.08/536,760. More specifically, one implements the practice of thepresent invention in such manner as to assure that, relative to thesurface energy retentive characteristics of one mold surface, that acertain effective differential be induced in the second mold surface, toeffect preferential displacement on a consistent basis throughout themanufacturing process. As aforesaid, it is understood that it ispreferred that the differential referred to favors release from theconvex surface, especially because the benefits of reduced lens defectsarising from poor wetting of the convex surface with RMM can therebyalso be realized.

The selection of surface modifying means is directly related to thewettability of the mold surface by the RMM as aforementioned. Thetheoretical estimation of wetting, which is the thermodynamic parametercalled the spreading coefficient, is defined as:

    S=γ.sub.s -γl.sup.- γ.sub.s l

where S is the spreading coefficient, γ_(s) is the surface energy of themold material, γl is the surface tension of the reactive monomer mix("RMM") and the γ_(sl) is the interfacial tension between reactionmonomer mix and the mold material. A positive "S" indicates spreading(or wetting). Therefore, in order to spread or to wet, γ_(l) and γ_(sl)should be made as small as possible or should be made as large aspossible. Practically, this means that the surface energy of the mold isincreased or the surface tension of the reactive monomer mix is reducedor both is performed in order to obtain the appropriate wetting betweenreactive monomer mix and the mold. With respect to increasing the moldsurface energy, this requires consideration of increasing not only thetotal surface energy, but also increasing the portion of high energysurface.

Thus, one method of affording improved wettability is to reduce surfacetension of the RMM by incorporating therein a surfactant (as disclosedin concurrently filed, commonly assigned Appln. Ser. No. 08/536,944 ofJames Jen et al. for METHOD FOR REDUCING LENS HOLE DEFECTS IN PRODUCTIONOF CONTACT LENS BLANKS, incorporated herein by reference. Another methodwith similar effect is to heat the RMM, which can reduce surface tensionfrom 38 dynes/cm at 20° C. to 34 dynes/cm at 60° C. which a concomitantdecrease is viscosity.

The mold surface when composed of polystyrene is highly non-polar, andhas a surface free energy of 42 dynes/cm. Wetting between the RMM andthe mold surface is alternatively improved by the presence of a surfacemodifying agent such as moisture at the interface in the form of a thinlayer. The surface energy of a water thin film is 72 dynes/cm hence asignificant increase of surface energy is readily achieved by coating athin water layer on the polystyrene mold surface. A uniform thinmoisture layer on the polystyrene mold surface represents a homogenoushigh energy, highly wettable surface. After assembly of the moldsurfaces about the RMM the water layer is absorbed by the RMM, renderingthe mold surface again less wettable and favoring lens demolding,emphasizing the importance of a transiently modified surface.

To correlate the improvement in lens hole defect performance to waterwettability, a modified Wilhelmy wetting method was developed to measurethe wettability of polystyrene backcurve molds. Water was used as theprobe liquid because of its high surface tension and its high polarity.The experimental detail is set forth in working Example 1.

FIG. 1, a plot of water wetting force vs lens hole defects, isillustrative. The plot shows that a mold surface with a water wettingforce greater than 70 mg produces little or no lens holes. A largervalue represents a large wetting force and indicates the mold surface ismore wettable. Among the findings was the observation that untreatedmold surfaces evidenced a large variation in wetting propertiesconsistently with the practical appreciation that lens hole defectsoccur on an apparently random, sporadic basis in production.

To determine the feasibility of surface treating mold material for usein the backcurve mold to reduce the tendency to form lens holes in thecontact lenses, the inventors developed a modified wetting method tomeasure the wettability of the backcurve mold. This method measures thewetting forces between the probe wetting liquid (D.I. water) and thebackcurve mold surface defined by the relationship:

    F=2γ cos 0

wherein

F is the wetting force (mg) of the mold half being measured;

γ1 is the surface tension of the probe liquid, i.e., the water;

p is the perimeter of the mold half at the meniscus (cm) when the moldhalf is partially immersed in the water; and

Θ is the dynamic contact angle in degrees 0.

In accordance with the invention, it has been determined that abackcurve with a water dynamic contact angle less than or equal to 100°produces contact lenses with significantly less lens holes. It is morepreferred that the contact angle be less than or equal to 90°; andparticularly preferred that such angle be less than or equal to 75°.

The surface modifying or wetting agent may be a surfactant; whereconsistent with the foregoing criteria, its selection is not critical,insofar as it is capable in wetting characteristics relative to RMM andin differential release characteristics relative to the companion moldsurface to which it is not applied to achieve the benefits of theinvention as outlined herein. Naturally, to the extent that thesurfactant is absorbed into the RMM, or remains to a certain extent onthe surface of the lens after hydration, it will be selected with regardto its physiological or pharmaceutical acceptability for human use ineye contact. The surface tension modifying characteristics of thesurfactant material in the circumstances obtaining will be assessedrelative to the desired differential release property; and the surfaceenergy modifying characteristics will be assessed relative to theenhanced wettability for the RMM in relation to the mold surface (inparticular, the convex mold surface, as aforesaid) in a manner wellknown to the artisan. The surfactant material is also selected forcompatibility with the mold materials and the reactive monomer mix.While, as a consequence, amounts to be applied may differ in response tothese characteristics, it has been found that in most cases, a selectedapplication in the range of 0.05 to 5.0 weight % of a solution of thesurfactant material is sufficient, usually 0.05 to 2.0 weight %.

Among otherwise suitable surfactants, anti-static agents, ionicsurfactants, non-ionic surfactants or lubricant formulations may beemployed in the present invention. Suitably, the surfactant constitutesa solution or dispersion of the surface active agents in an essentiallyinert vehicle to facilitate application to the mold surface by spraying,wiping, vapor deposition, sponging, dipping or the like. Thus, water, analkanol or mixtures thereof may be satisfactorily and economically usedto constitute a solution or dispersion of the surface active agent.

Among the materials found to not only aid in wettability of the moldsurface but also to retain effective release characteristics for lensdemolding from the frontcurve mold (where so applied) after hydrationand equilibration in saline is Tween 80 (registered trademark); i.e., aPolysorbate 80. This is basically polyethylene oxide sorbitanmono-oleate or the like equivalent, and consists of an oleate ester ofsorbitol and its anhydrides copolymerized with approximately 20 moles ofethylene oxide for each mole of sorbitol and sorbitol anhydrides, ofgenerally the formula: ##STR1##

Sum of w, s, y, z, is 20; R is (C₁₇ H₃₃)COO!

Generally, water soluble or water dispersible materials are preferredfor ease of application. As the mold materials are typicallymanufactured from hydrophobic materials such as polypropylene orpolystyrene, the wettability efficiency for these materials issignificant.

Other materials suitable for use include the pharmaceutically acceptableethoxylated amines and quaternary ammonium compounds such as Larostat264 A (a soy dimethyl ethyl ammonium ethosulfate sold by PPG), Armostat410 (an ethoxylated tertiary amine sold by Akzo), Cystat SN(3-lauramidopropyl trimethylammonium methyl sulfate sold by Cytec) andAtmer 163 (N,N-bis (2-hydroxyethyl) alkylamine)). Other quaternarycompounds include the diamidoamines, the imidazoliniums, the dialkyldimethyl quaternaries, the dialkoxy alkyl quaternaries, and themonoalkyl trimethyl quaternaries. Certain of these materials offer thefurther advantage of being soluble in RMM and hence are convenientlyresorbed into the lens material, leaving the mold surface unaffectedupon release and therefore more readily recycled for further use,without cleansing to a base, or neutral condition.

The surface active or wetting agent may be applied to the mold surfaceby spraying or swabbing or dipping, as aforesaid, such that the surfaceis evenly coated therewith. The mold is merely drip-dried with orwithout the aid of heat and stockpiled for use in the manufacturingprocess. The amount of surfactant so applied is adapted to provide auniform coating of 0.05 to 0.5 weight % solution of surfactant on thesurface of the mold, as aforesaid.

The application of the surface modifying or wetting agent mayalternately be suitably integrated with the manufacturing process suchthat the molds may be treated immediately prior to their interpolationwith the transport mechanism, or just prior to the filling operation,such that the surface thereof is fully wetted, i.e., undried at thepoint of contact with the reactive monomer mixture.

The molds can be made from any thermoplastic material which is suitablefor mass production and can be molded to an optical quality surface andwith mechanical properties which will allow the mold to maintain itscritical dimensions under the process conditions employed in the processdiscussed in detail below, and which will allow polymerization with theinitiator and radiant energy source contemplated. The concave and convexmold members can thus be made from thermoplastic resins. Examples ofsuitable materials include polyolefins such as low, medium, and highdensity polyethylene, polypropylene, including copolymers thereof;poly-4-methylpentene; and polystyrene. Other suitable materials arepolyacetal resins, polyacrylethers, polyarylether sulfones, nylon 6,nylon 66 and nylon 11. Thermoplastic polyesters and various fluorinatedmaterials such as the fluorinated ethylene propylene copolymers andethylene fluoroethylene copolymers may also be utilized.

It has been found that with the need for a high quality, stable mold andespecially for the use of a plurality of molds in high volume operationsthe choice of material for the molds is significant. In the presentinvention the quality of production is not assured by individualinspecting and sorting each lens for power and curvature. Instead thequality is assured by keeping the dimensions of each individual moldmember within very tight tolerances and processing molds in particularsequential steps to give all lenses equal treatment. Since polyethyleneand polypropylene partly crystallize during cooling from the melt thereis a relatively large shrinkage giving dimensional changes difficult tocontrol. Thus, it further has been found that the most preferredmaterial for the molds used in the present process is polystyrene whichdoes not crystallize, has low shrinkage, and can be injection molded atrelatively low temperature/to surfaces of optical quality. It will beunderstood that other thermoplastics, including those mentioned above,may be used provided they have these same properties. Certain copolymersor blend of polyolefins that exhibit these desirable characteristics arealso suitable for the present purposes as are polystyrene copolymers andblends having such characteristics, as described more fully in U.S. Pat.No. 4,565,348.

The soft contact lens blanks are formed from a reactive monomercomposition which typically incorporates in addition to the reactivemonomer a water displaceable diluent in the case of the preparation of ahydrophilic lens, a polymerization catalyst to assist in curing thereactive monomer, a cross-linking agent and often a surfactant to aid inmold release.

The curable compositions preferably include copolymers based on2-hydroxyethyl methacrylate ("HEMA") and one or more comonomers such as2-hydroxyethyl acrylate, methyl acrylate, methyl methacrylate, vinylpyrrolidone, N-vinyl acrylamide, hydroxypropyl methacrylate, isobutylmethacrylate, styrene, ethoxyethyl methacrylate, methoxytriethyleneglycol methacrylate, glycidyl methacrylate, diacetoneacrylamide, vinyl acetate, acrylamide, hydroxytrimethylene acrylate,methoxyethyl methacrylate, acrylic acid, methacryl acid, glycerylmethacrylate, and dimethylamino ethyl acrylate.

Preferred polymerizable compositions are disclosed in U.S. Pat. No.4,495,313 to Larsen, U.S. Pat. No. 5,039,459 to Larsen et al. and U.S.Pat. No. 4,680,336 to Larsen et al., the disclosures of which are herebyincorporated herein by reference. Such compositions comprise anhydrousmixtures of a polymerizable hydrophilic hydroxy ester of acrylic acid, adisplaceable ester of boric acid and a polyhydroxyl compound havingpreferably at least 3 hydroxyl groups. Polymerization of suchcompositions, followed by displacement of the boric acid ester withwater, yields a hydrophilic contact lens. The mold assembly utilized inthe present invention may be employed to make hydrophobic or rigidcontact lenses, but the manufacture of hydrophilic lenses is preferred.

The polymerizable compositions preferably contain a small amount of across-linking agent, usually from 0.05 to 2% and most frequently from0.05 to 1.0%, of a diester or triester. Examples of representative crosslinking agents include: ethylene glycol diacrylate, ethylene glycoldimethacrylate, 1,2-butylene dimethacrylate, 1,3-butylenedimethacrylate, 1,4- butylene dimethacrylate, propylene glycoldiacrylate, propylene glycol dimethacrylate, diethylglycoldimethacrylate, glycol dimethacrylate, diethylglycol dimethacrylate,dipropylene glycol dimethacrylate, diethylene glycol diacrylate,dipropylene glycol diacrylate, glycerine trimethacrylate, trimethylolpropane triacrylate, trimethylol propane trimethacrylate, and the like.Typical cross-linking agents usually, but not necessarily have at leasttwo ethylenically unsaturated double bonds.

The polymerizable compositions generally also include a catalyst,usually from about 0.05 to 1% of a free radical catalyst. Typicalexamples of such catalysts include lauroyl peroxide, benzoyl peroxide,isopropyl percarbonate, azobisisobutyronitrile and known redox systemssuch as the ammonium persulfate-sodium metabisulfite combination and thelike. Irradiation by ultraviolet light, electron beam or a radioactivesource may also be employed to catalyze the polymerization reaction,optionally with the addition of a polymerization initiator.Representative initiators include camphorquinone,ethyl-4-(N,N-dimethyl-amino)benzoate, and4-(2-hydroxyethoxy)phenyl-2-hydroxyl-2-propyl ketone.

Polymerization of the polymerizable composition in the mold assembly ispreferably carried out by exposing the composition to polymerizationinitiating conditions. The preferred technique is to include in thecomposition initiators which work upon expose to ultraviolet radiation;and exposing the composition to ultraviolet radiation of an intensityand duration effective to initiate polymerization and to allow it toproceed. For this reason, the mold halves are preferably transparent toultraviolet radiation. After the precure step, the monomer is againexposed to ultraviolet radiation to a cure step in which thepolymerization is permitted to proceed to completion. The requiredduration of the remainder of the reaction can readily be ascertainedexperimentally for any polymerizable composition.

The mold assembly comprises at least two pieces, a female concave piece(frontcurve) and a male convex piece (backcurve), forming a cavitytherebetween, and when said pieces are mated, at least one piece havinga flange thereabout. More particularly, the mold assembly comprises afront mold half and a back mold half in contact therewith, therebydefining and enclosing a cavity therebetween, and a polymerizablecomposition in said cavity in contact with said mold halves, the frontmold of which has a central curved section with a concave section with aconcave surface, a convex surface and circular circumferential edge,wherein the portion of said concave surface in contact with saidpolymerizable composition has the curvature of the frontcurve of acontact lens to be produced in said mold assembly and is sufficientlysmooth that the surface of a contact lens formed by polymerization ofsaid polymerizable composition in contact with said surface is opticallyacceptable, said front mold also having an annular flange integral withand surrounding said circular circumferential edge and extendingtherefrom in a plane normal to the axis and extending from said flange,while the back mold has a central curved section with a concave surface,convex surface and circular circumferential edge, wherein the portion ofsaid convex surface in contact with said polymerizable composition hasthe curvature of the backcurve of a contact lens to be produced in saidmold assembly and is sufficiently smooth that the surface of a contactlens formed by polymerization of the polymerizable composition incontact with said surface is optically acceptable, said backcurve alsohaving an annular flange integral with and surrounding said circularcircumferential edge and extending therefrom in a plane normal to theaxis of said convex structure, and a generally triangular tab situatedin a plane normal to said axis and extending from said flange, whereinthe convex structure of said back mold half contacts the circumferentialedge of the front mold half.

The inner concave surface of the front mold half defines the outersurface of the contact lens, while the outer convex surface of the basemold half defines the inner surface of the contact lens which rests uponthe edge. Specifics of such constructions are known having reference toU.S. Pat. No. 4,640,489 to Larsen; improvements are shown in U.S. Ser.No. 08/258,654 to Martin et al., incorporated herein by reference.

In applying the surface active material to the surface of the moldseveral considerations should be borne in mind: a sufficient level ofsurfactant is employed to uniformly cover the mold surface to effect thedesired enhancement of wettability in the circumstances obtaining, theagent is retained on the surface in effective amounts for use in thefilling operation, and where absorption into the RMM occurs, the amountof material employed is not in excess of that which can be diffused intoand absorbed by the RMM prior to the solidification of the lens.

As is self evident, the wetting agent is pharmaceutically acceptable atthe levels employed, and does not affect the surface or the efficacy ofthe finished lens.

In accordance with the invention, a convex mold surface which has beenpretreated with the surface modifying agent, as aforesaid is broughtinto mating engagement with a frontcurve mold and the reactive monomermix filling the molding cavity therebetween, during which process theoptical surface of the convex mold contacts and by reason of itsmodified surface is efficiently wetted by the RMM such that theadvancing meniscus uniformly coats the mold surface without theformation of lens hole defects. The convex mold, when mechanicallyseparated from the concave mold by reason of its modified surface,efficiently (and consistently throughout the process, involving aninteraction of similarly treated molds) releases the lens blank (whichremains with the concave mold for curing) without tearing or otherwisedamaging the lens blank.

In a preferred embodiment, the frontcurve mold is formed from acomposition which comprises an added mold release agent such as zincstearate, which aids in demolding of the lens blank after hydration, asdisclosed in copending and commonly assigned U.S. Ser. No. 414,999,incorporated herein by reference. In consequence, its surface energycharacteristics have been modified and an additional level of surfacemodifying agent (in this case a combination of water vapor andsurfactant application) may be required in application to the convexmold surface to balance the release characteristics in such manner as toassure release of the lens blank for retention by the concave orfrontcurve mold surface.

It will be understood in connection with the foregoing description thatthe concepts as well as theoretical considerations affecting demoldingare entirely distinct from the problem of adequately wetting the surfaceof the forming mold with the monomer composition although eachconsideration is interrelated to the other in practice, i.e., one maysuccessfully demold a lens blank which, however, is defective due to oneor more lens holes. The contact portion of the convex lens mold inparticular must be receptive to the monomer composition at the point andtime of application under the conditions then obtaining, in the sensethat it requires a critical wettability controlling efficientspreadability across the contact surface of the mold to be established,by increasing the surface area comprising high surface energy domains inthe convex mold contact surface.

While the invention has been described with particular reference toapplication of the surface modifying agent to the optical surface of theconvex mold in a particular manufacturing operation, including agenerally vertical disposition of the mating mold elements with theconcave member generally supporting the incipient lens blank in a supineor lower position, it will be understood that to effect preferentialdisplacement of the lens blank as, for example, in other geometricarrangements the surface modifying agent may be applied to the opticalsurface of either lens mold surface.

Also, while the present invention has been primarily illustrated byreference to the control of a wettable surface on the convex moldoptical surface to control lens hold defects, it is also effective inminimizing puddling on the concave lens surface facilitating the initialspreading and coating of the surface by the RMM in a uniform, homogenousmanner.

EXAMPLE 1

A number of treated, backcurve molds were tested for water wetting forceas follows: The treated mold is suspended vertically on a microbalanceabove a body of water functioning as a probe liquid. The water level israised incrementally to immerse the backcurve. The wetting force betweenthe probe liquid and the backcurve as the probe liquid is being raisedis measured by the microbalance and recorded. A trace of the wettingforces as a function of distance travelled by the water meniscus overthe backcurve is obtained from which the value p (the perimeter of themold half at the meniscus (cm) when the mold half is partially immersedin the water) can be calculated which when taken together with thewetting force (mg) F can be employed to determine the dynamic contactangle in degrees according to the formula:

    F=2γ cos Θ

where γ is the surface tension of water (72.75 dynes/cm at 20° C.). Asdescribed hereinabove, it is preferred that the water dynamic contactangle afforded by the treated surface is less than or equal to 100°,preferably less than or equal to 90°.

The apparent water wetting force for various treated and untreatedbackcurves is set forth as follows in Table 1.

                  TABLE 1                                                         ______________________________________                                        MOLD SURFACE        WETTING FORCE                                             ______________________________________                                        Untreated Polystyrene A                                                                           56.4                                                      Untreated Polystyrene B                                                                           56.9                                                      Untreated Polystyrene C                                                                           50.9                                                      0.25% CYASTAT LS*/Polystyrene B                                                                   67.1                                                      0.5% CYASTAT LS/Polystyrene B                                                                     113.0                                                     0.05% CYASTAT LS/Polystyrene C                                                                    105.0                                                     0.25% CYASTAT LS/Polystyrene C                                                                    81.0                                                      0.5% CYASTAT LS/Polystyrene C                                                                     115.0                                                     ______________________________________                                         *(3-lauramidopropyltrimethylammonium methyl sulfacte, sold by Cytec           Industries)                                                              

EXAMPLE 2

Convex molds were immersed in 2% aqueous solutions of Larostat 264 A(PPG), Armostat 410 (Akzo) and Cystat SN (Cytec), respectively, and thendried under ambient conditions for 48 hours. The thus formed coatingrendered the surface more wettable by the RMM, a HEMA based composition.

When the treated molds were employed in an automated pilot manufacturingfacility, lens mold defects were reduced by about 34.6%.

EXAMPLE 3

Polystyrene contact lens molds were swabbed with Glucam P-10, Tween 80and an aqueous dispersion of Glucam DOE 120, respectively, and the moldswere utilized in molding contact lenses employing a reactive monomer mixcomprising 96.8% HEMA, 1.97% methacrylic acid, 0.78% ethylene glycoldimethacrylate, and 0.1% of trimethylolpropane trimethacrylate and 0.34%of Darvocur 1173 dispered (48% RMM) in glycerin boric acid ester as aninert water displaceable diluent. The mold halves were readily separatedwithout defects.

Aspects of the invention described herein are related to subject matterdisclosed and claimed in concurrently filed Appln. Ser. No. 08/536,944of James Jen et al. for METHOD FOR REDUCING LENS HOLE DEFECTS INPRODUCTION OF CONTACT LENS BLANKS incorporated herein by reference.

What is claimed is:
 1. In a process for molding hydrophilic hydrogelcontact lenses from a reactive monomer mixture in which cooperatingconcave and convex mold faces are interposed in molding relation about acavity formed therebetween to form a contact lens blank, containingreactive monomer mixture the improvement which comprises treating atleast one of said mold faces with a transient coating of an agentcomprising condensed water vapor to facilitate wetting of said mold facewith the reactive monomer mixture whereby lens hole defects and orpuddle defects in said contact lens blanks are reduced.
 2. The processof claim 1 wherein said agent further comprises a surfactant effectiveto render more homogeneous the high surface energy domains present onthe mold surface.
 3. The process of claim 2 wherein said agent isabsorbable by said reactive monomer mixture.
 4. The process of claim 1wherein the convex backcurve is pretreated with the wetting agent. 5.The process of claim 1 wherein the mold is constructed of a hydrophobicmaterial.
 6. The process of claim 5 wherein said hydrophobic material inpolystyrene.
 7. The method of claim 1 wherein the wetting agent asapplied to the optical mold surface exhibits a water wetting force of atleast 70 mg.
 8. The method of claim 1 wherein the treated opticalsurface of the convex mold exhibits a water dynamic contact angle ofless than or equal to 100°.
 9. In a process for molding contact lensblanks including filling a concave lens mold with a reactive monomermixture, juxtaposing therewith in mating molding relation a convex lensmold, the improvement which comprises pretreating at least one of theoptical surfaces of said molds with water vapor to condense an evencoating of moisture thereon to enhance wettability of the opticalsurface to the reactive monomer mix.
 10. In a process for moldinghydrophilic hydrogel contact lenses from a reactive monomer mixture inwhich cooperating concave and convex mold faces are interposed inmolding relation about a cavity formed therebetween to form a contactlens blank, containing reactive monomer mixture the improvementcomprising maintaining a relative humidity of from about 60% to about80% in the cavity.